1. Field of the Invention
The present invention relates to a signal amplifier.
2. Description of the Related Art
A so-called Digitally Controlled Power Amplifier (DPA) capable of controlling an amplified amplitude by discrete values is known as a signal amplifier that is used for amplifying an amplitude modulation component (a change component of an envelope) of a radio-frequency signal (hereinafter, RF signal) that is subjected to modulation including an amplitude modulation. For example, JP-A. 2009-89225 (KOKAI) proposes the DPA having a differential output configuration as one of various DPAs, which is capable of performing amplification with high linearity with respect to a wide range of a signal amplitude level.
The DPA proposed in JP-A. 2009-89225 (KOKAI) operates while maintaining the number of MOS transistors in an ON operation state and the number of MOS transistors in an OFF operation state of each of a plurality of MOS transistors on an in-phase side and a reversed phase side that are controlled to be in the ON operation state and the OFF operation state by a control signal based on a complementary Amplitude Control Word (ACW), in a predetermined relationship.
However, there is an unavoidable problem in the MOS transistor that the operation characteristics degrade due to aging. Therefore, when the conventional DPA proposed in JP-A. 2009-89225 (KOKAI) is kept operating in the above operation state, the above described predetermined relationship is disrupted due to the aging of the MOS transistor, so that symmetry between output signals on the normal phase side and the reversed phase side is disrupted. For example, even if a differential output is controlled at zero level by the ACW, an output level does not become the zero level and has a value. In other words, with the above described conventional DPA, the linearity of an output amplitude with respect to the ACW is disrupted due to the aging, so that a correct output cannot be obtained.